Most of the time when there is a serious breakup and even a friendly parting of the ways, it is pretty devastating and can be a very distressing situation for you, no matter what the reason is for the breakup.
These three things are just a few suggestions for you...
Be honest with you ex and yourself- I hate to keep repeating this, but need to.
Honesty is the best policy as they say; and it really is true here.
If you have done something that has caused your ex to leave you, then you really need to be straight out honest with them about why you did what you did and ask if they can ever forgive you for what you have done and for the pain that your actions have caused them.
This is a place to start.
Ask for forgiveness for your mistakes- Although this might seem to be pretty simple; it really isn't for many people simply because they do not like admitting that they messed up that badly and/or they do not like feeling vulnerable like that.
If you are the one whose action and/or attitude caused the end of the relationship, you might not want to "bear your soul" so to speak - or take the chance that you ex will shoot you down without hearing you out.
Ask your ex to help you work through these things- Now this one might not sound like a winner in this department.
If you are willing to admit your mistakes, ask their forgiveness for the damage done, your ex will see that you are willing to change and may choose to help you to make that change.
You also need to make sure that you are ready for the fact that your ex may not be able to help you.